The optical inspection of semiconductor wafers for defects is an important part of the manufacturing process for computer chips. In particular, for examining the upper and lower sides of flat wafers a dark field illumination is employed, for visibilizing the irregularities. This facilitates the search for defects such as scratches, breaks, cracks, or impressions in the surface, or particles on the surface, of the wafer. In the context of the present document, the term “irregularity” is used to refer to any type of defect(s). The desired inspection enables not just a qualitative but also a quantitative evaluation of the surface quality. Accordingly, an inspection device of the general type described supra should be capable of achieving a very fine detection, such that characteristics of each defect type can be determined and the defects found can be classified.
Numerous inspection devices are known which employ various means to achieve the stated objective. In U.S. Pat. No. 7,123,357 B2, for example, various methods are disclosed for classification of surface defects in dark field measurement, employing a plurality of laser beams oriented mutually orthogonally, which beams cross at the object surface, preferably in the radial and circumferential directions of the wafer. The intensities of the scattered light from different laser beams are detected separately and are compared with each other, to garner information about the anisotropy, orientation, and aspect ratios of defects.
U.S. Pat. No. 6,956,644 B2 describes a method of optical surface inspection of wafers in the dark field wherein the surface is illuminated pointwise with two different angles of incidence of the light and/or with two different wavelengths, from a laser, one after the other. The scattered light from the different illumination situations is received by a respective detector, and from the differences in intensity of the scattered light it is concluded whether a detected anomaly on the object surface is a particle on the surface or a so-called “crystal originated particle” (COP). The term COP is historical and it is misleading in that it signifies a defect in the wafer surface and not an actual separate particle.
In U.S. Pat. No. 7,061,598 B2, the inventors take a different approach. In the method for dark field inspection of a wafer surface, an optical element is used to deflect light from a laser which is scattered from a wafer surface (in particular, from an irregularity on the surface), wherewith the light is deflected onto a location-resolving detector such that information about intensity and also scattering angle is obtained which information reveals keys to the type of the defect.
In U.S. Pat. No. 7,304,310 B2, a method is proposed wherein UV light is selectively guided in combination with light of a different wavelength onto a substrate surface of the object sought to be examined. In the dark field inspection the scattered light is measured at different detection angles in combination with different detection wavelengths. In this way it is said that one can distinguish between scattered light and fluorescence light, and one can draw conclusions about the source (on the substrate surface) of the scattered light.
A problem confronting all inspection methods with dark field illumination is that different types of defects have very different physical scattering behavior. This relates to, first, the intensity of the scattered light, which is different for small-surface (point-like) defects or particles and large-surface defects such as scratches or breaks. Secondly, anisotropic defects (which have a primary direction), e.g. scratches or cracks cause the scattered signal to depend sharply on the relative arrangement between the scattered light detector and the light source. Finally, defects having dimensions less than one half of the wavelength of the light used lead, via interference, to a substantial spatial modulation of the scattered light (speckles) and therefore again to strong dependence of the measurement signal on the relative arrangement.
General solutions to reduce such artifacts are disclosed in, e.g., U.S. Pat. No. 6,788,404 B2. In that patent it is proposed to combine a plurality of beams of light sources of different frequencies, in particular a broadband and a narrow-band light source, into a single light beam, and to direct this combined beam to the surface to be examined. In this way, the overall intensity of the light is increased, but interference effects are reduced by the use of the broadband light source, while at the same time the high intensity of the narrow-band light component is exploited.
Another proposed solution is disclosed in the patent EP 1257869 B1. This relates to a device and a method for reducing the abovementioned “laser speckles” when illuminating rough surfaces. With this solution, polarization effects are eliminated by dividing an emitted laser beam into two partial beams of equal intensity but orthogonal polarization. The partial streams are passed through optical paths with oscillating path length differences, and are then collected into a common beam with which the surface is illuminated.
Apart from the fact that the latter two proposed solutions require complex and costly measures relating to the illumination device, they only partly solve the set of problems presented relating to the scattered light.